1. Field of the Invention
The present invention relates generally to the field of fuel cells and, more specifically, to a direct methanol fuel cell system in which active control of the concentration of methanol at a critical point within the cell allows dynamic response to changes in power demand while minimizing crossover of methanol through the cell""s membrane.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suitable for use as a fuel, depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or formaldehyde, are attractive choices for fuels due to their high specific energies.
Fuel cell systems may be divided into xe2x80x9creformer basedxe2x80x9d (i.e., those in which the fuel is processed in some fashion before it is introduced into the cell) or xe2x80x9cdirect oxidationxe2x80x9d in which the fuel is fed directly into the cell without internal processing. Most currently available fuel cells are of the reformer-based type, and their fuel processing requirement limits their application to relatively large applications relative to direct oxidation systems.
An example of a direct oxidation system is the direct methanol fuel cell system or DMFC. In a DMFC, the electrochemical reaction at the anode is a conversion of methanol and water to CO2, H+ and exe2x88x92. The hydrogen ions flow through a membrane electrolyte to the cathode, while the free electrons flow through a load which is normally connected between the anode and cathode. At the cathode, oxygen reacts with hydrogen ions and free electrons to form water.
In addition, conventional DMFCs suffer from a problem which is well known to those skilled in the art: cross-over of methanol from the anode to the cathode through the membrane electrolyte, which causes significant loss in efficiency. Cross-over occurs because of the high solubility of methanol in the membrane electrolyte. In order to minimize cross-over, and thereby minimize the loss of efficiency, the concentration of methanol in the fuel feed stream is kept low (e.g., below 1M) by dilution with water. However, dilution of the methanol introduces other disadvantages: (1) the fuel cell""s construction becomes more complicated and costly because of the structures and processes needed to store and manage the water; and (2) the energy per unit volume of the fuel cell, which is a critical factor in terms of the fuel cell""s potential commercial applications, is reduced.
Many important applications of DMFCs require that the power source be able to vary its output in response to constantly changing electrical loads. There is a direct relationship between the electric load and the amount of methanol consumed. As such, it is desirable to vary the amount of methanol that is fed into the fuel cell, to supply slightly more fuel to the fuel cell than is consumed by the reaction that generates electricity. However, supplying substantially more fuel than is required has several disadvantages: it increases methanol crossover, which decreases efficiency of the system; in an open anode configuration, it increases the amount of fuel that passes through without reacting, thus wasting fuel; and in a closed anode configuration, it increases the volume of unreacted fuel that must be recirculated, consuming energy and increasing the demands on the system.
It is known to those skilled in the art that conventional efforts to actively control the methanol concentration, for the purposes of regulating the power output of a DMFC system and minimizing cross-over, have been accompanied by another disadvantage. In order to effect control using conventional methods, a methanol concentration sensor must be provided, the presence of which tends to further increase the cost and complexity of the fuel cell system as well as introduce an additional component whose failure could significantly affect performance.
The present invention provides a direct methanol fuel cell system in which the concentration of methanol supplied to the anode is actively controlled so as to optimize the performance of the system, and minimize cross-over, but without the need for a methanol concentration sensor. The present invention monitors one or more of the fuel cell""s operating characteristics, such as the potential across the load, the open circuit potential or the short circuit current. A preferable choice of operating characteristic is one for which a known relationship exists between that characteristic and methanol concentration. In this fashion, the monitored characteristic may be used, directly or indirectly, to generate a control signal which is supplied to a regulator. The regulator, which is coupled to sources of methanol and water and responsive to the control signal, functions to increase or decrease the concentration of methanol supplied to the anode.
In a first embodiment, the potential across the load or fuel cell stack is detected. Periodically, the flow of methanol is reduced or interrupted. As the methanol concentration drops, the potential across the load drops. By measuring the time required for the potential to drop to a predetermined value or the rate at which the potential drops, an initial methanol concentration may be determined and used to calculate or retrieve from a memory appropriate values for adjusting the concentration.
In a second embodiment, the potential is detected at a portion of the anode which is proximate to the end of the methanol flow path. Because the methanol concentration is normally lowest at the end of the flow path, the proximate portion of the anode is highly sensitive to changes in methanol concentration. The detected potential is compared to a predetermined reference or to the potential at another point of the anode which is upstream in the flow path, the difference being used to adjust the concentration.
In a third embodiment, the load is periodically disconnected from the fuel cell, thereby creating brief periods of time in which the open circuit potential of the fuel cell is detected. The detected potential is stored and compared to a previously stored value which represents the detected potential at an earlier time or a predetermined reference. Because the fuel cell""s open circuit potential decreases as a result of methanol cross-over, the difference in the compared potentials may be used to adjust the methanol concentration.
In a fourth embodiment, the load is periodically short-circuited, thereby creating is brief periods of time in which the short circuit current of the fuel cell is detected and stored. Because all of the current produced by the fuel cell passes through the short circuit, the stored value serves as an indicator of how much methanol is available to the anode at a given time. The stored value is compared to a reference and the difference used to adjust the methanol concentration.